An oxidative autothermal reformer including a reforming layer at least partially filled with a reforming catalyst for producing a reformed gas composed mainly of hydrogen through a reforming reaction by contacting a mixture of a hydrocarbon or an aliphatic alcohol and steam with the reforming catalyst; and an oxidative exothermic layer at least partially filled with an oxidation catalyst for generating heat by oxidizing a part of the reformed gas, in which the reforming layer is disposed at an upstream side of the oxidative exothermic layer; the reforming layer and the oxidative exothermic layer are in the form of a cylinder and have a triple circular tube structure formed by disposing an inner reforming layer, an oxidative exothermic layer and an outer reforming layer from the inside in a radial direction in this order; and at least a part of the reforming catalyst filled in the inner reforming layer and the outer reforming layer contains Ru metal.